It's possible. The mucus plug-like thing you saw could be a sign of implantation.
The other possibility though (among others that are less exciting, I guess), is that you could be dealing with perimenopause. Since I don't know your age, this could be totally off! But for most women perimenopause, which is the stage before true menopause, starts in your 30-40's and can cause late periods and all of the fun that comes with that.
Your best bet right now is to grab a pregnancy test and take it. Since you're already late, it doesn't matter what time of day you take it. Although rare, according to americanpregnancy.org, between 2-10 women out of every 1,000 will become pregnant after 5+ years following their tubal ligation. So, even though I bet this isn't what's going on with you, there is that slim possibility that inspires a pregnancy test purchase.